A device as described in the opening paragraph is already known from Japanese Patent Application JP-3-192801-A of 21-12-1989, MITSUBISHI ELECTRIC CORP., Application no. 1-333616, published 22-08-1991). This device comprises a microwave transistor stage (1) which has a microwave input (4), an output (2) via the drain of the transistor whose source is connected to ground, a microwave matching circuit formed by transmission lines (3) and (5) connected to the gate of the transistor (1), and a d.c. bias circuit (6) connected to the matching circuit (3, 5) at the node (13) at the end of the line (5). The d.c. bias circuit first of all comprises a bias resistor (9) connected between the link node (13) and the d.c. power supply (11) and a decoupling capacitor (10) of the d.c. power supply.
The bias circuit (6) disclosed in the cited Japanese Patent Application further includes a short line (19) connected to the link node (13), which line is provided to form a low-value capacitor capable of forming, together with the bias resistor (9), an RC filter which also takes into account the value of the d.c. voltage (11) decoupling capacitor (10).
Thus, in the circuit disclosed in said document the bias resistor (9) is always to be taken into account in the microwave operation. In this frequency domain this resistor cooperates with the capacitors (19, 10) to form a filter.
As disclosed in the Japanese document reflection coefficient or mismatch factor at the operating frequency is close to 1 when viewing the bias circuit (6) from the link node (13). Thus, the possibility for oscillations to appear is diminished.
However, this known circuit has certain drawbacks. First of all, obtaining a mismatch factor (reflection coefficient) close to 1, but nevertheless with a slightly negative factor, is not sufficient to insure stability. As long as a circuit has a negative mismatch factor, the possibility of the occurrence of severe oscillations is not negligible.
On the other hand, the open stub element (19) which is used has a filter function centered on a narrow band. Outside this band, annoying oscillations can be foreseen at lower or higher frequencies.